Patent application title: ORGANIC ELECTROLUMINESCENCE DEVICE AND MULTI-COLOR DISPLAY APPARATUS USING THE SAME

Abstract:

Provided is an organic electroluminescence device which includes an
inorganic protective layer having sufficient device protection
performance and high light extraction efficiency and which is excellent
in water resistance, acid resistance, and mechanical strength. The
organic electroluminescence device satisfies:
[{(2m+1)/4}-(1/8)]λ<nd<[{(2m+1)/4}+(1/8)]λ
where d indicates a thickness of an inorganic protective layer, n
indicates a refractive index of the inorganic protective layer, λ
indicates a maximum peak wavelength of a spectrum of light emitted from
the organic electroluminescence device, and m indicates a natural number.

Claims:

1. An organic electroluminescence device, comprising: a first electrode;
a second electrode; an organic compound layer which is provided between
the first electrode and the second electrode and includes an emission
layer; and a first inorganic protective layer on the second electrode,
wherein a thickness of the first inorganic protective layer satisfies:
[{(2m+1)/4}-(1/8)]λ<nd<[{(2m+1)/4}+(1/8)]λ where d
indicates the thickness of the first inorganic protective layer, n
indicates a refractive index of the first inorganic protective layer,
λ indicates a maximum peak wavelength of a spectrum of light
emitted from the organic electroluminescence device, and m indicates a
natural number.

2. The organic electroluminescence device according to claim 1, wherein
the first inorganic protective layer comprises one of SiN and TiO.sub.2.

3. The organic electroluminescence device according to claim 1, wherein
the natural number m is 1.

4. The organic electroluminescence device according to claim 1, further
comprising: a coverage layer made of a resin, which is provided on the
first inorganic protective layer, and has a thickness of 5 μm or more
to 50 μm or less; and a second inorganic protective layer which is
different from the first inorganic protective layer and provided on the
coverage layer, and has a thickness of 0.5 μm or more to 3 μm or
less.

5. A multi-color display apparatus for at least two colors, comprising
multiple organic electroluminescence devices each comprising the organic
electroluminescence device according to claim 1.

6. The multi-color display apparatus according to claim 5, wherein: the
first inorganic protective layer which is in contact with the second
electrode is provided across the multiple organic electroluminescence
devices at a common thickness; and the common thickness of the first
inorganic protective layer satisfies:
[{(2m1+1)/4}-(1/8)]λ1<n1d1<[{(2m1+1)-
/4}+(1/8)]λ1 where d1 indicates the common thickness,
n1 indicates a refractive index of the first inorganic protective
layer which is in contact with the second electrode, λ1
indicates a maximum peak wavelength of a spectrum of light emitted from
an organic electroluminescence device having lowest light emitting
efficiency, of the multiple organic electroluminescence devices, and
m1 indicates a natural number.

Description:

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an organic electroluminescence
(hereinafter, referred to as "EL") device applied to a flat panel
display, a projection display, and an illumination device, and to a
multi-color display apparatus using the organic EL device.

[0003] 2. Description of the Related Art

[0004] Organic EL devices using electroluminescence of an organic material
have been actively researched and developed. Of the organic EL devices, a
top emission type organic EL device (in which substrate, reflective
electrode, organic layer, and transparent electrode are laminated in
order to emit light in this lamination direction) capable of preventing
aperture ratio loss caused by wirings and thin film transistors (TFTs) is
becoming mainstream.

[0005] However, an organic EL material is sensitive to moisture, and hence
a structure in which a protective layer is formed has been proposed to
improve reliability. A protective layer which is located on an upper
electrode and contains silicon, oxygen, or nitrogen is disclosed in
Japanese Patent Application Laid-Open No. H07-161474. In order to improve
light extraction efficiency in the top emission type organic EL device, a
technology of defining a refractive index and thickness of an organic
capping layer located on an upper electrode is disclosed in Japanese
Patent Application Laid-Open No. 2006-156390.

[0006] However, when a thickness of the protective layer disclosed in
Japanese Patent Application Laid-Open No. H07-161474 is to be set to
enhance light having a desired wavelength, to thereby obtain an effect of
improving the light extraction efficiency as described in Japanese Patent
Application Laid-Open No. 2006-156390, sufficient protection performance
may not be obtained because the protective layer is too thin.

SUMMARY OF THE INVENTION

[0007] An object of the present invention is to provide an organic EL
device which includes an inorganic protective layer having sufficient
device protection performance and high light extraction efficiency and
which is excellent in water resistance, acid resistance, and mechanical
strength, and a multi-color display apparatus using the organic EL
device.

[0008] According to the present invention, there is provided an organic
electroluminescence device, including: a first electrode; a second
electrode; an organic compound layer which is provided between the first
electrode and the second electrode and includes an emission layer; and a
first inorganic protective layer on the second electrode, in which a
thickness of the first inorganic protective layer satisfies:

[{(2m+1)/4}-(1/8)]λ<nd<[{(2m+1)/4}+(1/8)]λ

where d indicates the thickness of the first inorganic protective layer,
n indicates a refractive index of the first inorganic protective layer,
λ indicates a maximum peak wavelength of a spectrum of light
emitted from the organic electroluminescence device, and m indicates a
natural number.

[0009] According to the present invention, the thickness of the inorganic
protective layer is defined based on a light emitting wavelength to
further improve a microcavity of the EL device. Therefore, when the
inorganic protective layer which is excellent in moisture resistance,
acid resistance, and mechanical strength is used, there may be provided
the organic EL device having high reliability and excellent light
extraction efficiency, and the multi-color display apparatus using the
organic EL device.

[0010] Further features of the present invention will become apparent from
the following description of exemplary embodiments with reference to the
attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] FIG. 1 is a schematic cross sectional view illustrating an organic
EL device according to a preferred embodiment of the present invention.

[0012]FIG. 2 is a schematic cross sectional view illustrating a
multi-color display apparatus according to the preferred embodiment of
the present invention.

DESCRIPTION OF THE EMBODIMENTS

[0013] An organic EL device according to the present invention includes a
first electrode, a second electrode, an organic compound layer which is
located between the first electrode and the second electrode and includes
an emission layer, and an inorganic protective layer which is in contact
with the second electrode and provided on a side opposite to the first
electrode side. A thickness of the inorganic protective layer satisfies:

[{(2m+1)/4}-(1/8)]λ<nd<[{(2m+1)/4}+(1/8)]λ

where d indicates the thickness of the inorganic protective layer, n
indicates a refractive index of the inorganic protective layer, λ
indicates a maximum peak wavelength of a spectrum of light emitted from
the organic electroluminescence device, and m indicates a natural number.

[0014] Hereinafter, the organic EL device according to the present
invention is described with reference to FIG. 1 illustrating a schematic
cross sectional view of an embodiment. The organic EL device illustrated
in FIG. 1 has a top emission type structure in which a reflective
electrode 2, a hole transport layer 3, an emission layer 4, an electron
transport layer 5, an electron injection layer 6, a translucent electrode
7, a first inorganic protective layer 8, a coverage layer 9, and a second
inorganic protective layer 10 are provided in this order on a substrate
1. In this embodiment, the layers from the hole transport layer 3 to the
electron injection layer 6 constitute an organic compound layer 11. The
reflective electrode 2 corresponds to the first electrode in the present
invention. The translucent electrode 7 located on a light extraction side
corresponds to the second electrode in the present invention. In the
organic EL device, a voltage is applied between the reflective electrode
2 and the translucent electrode 7 to supply a current to the organic
compound layer 11. Therefore, holes and electrons which are injected from
the respective electrodes are recombined in the emission layer 4 to emit
light. The reflective electrode is an electrode in which a reflectance on
a surface thereof in a visible light range (400 nm to 780 nm in
wavelength) is equal to or larger than 50%. The translucent electrode is
an electrode in which the reflectance in the visible light range is equal
to or larger than 40%.

[0015] An organic EL device generally has a microcavity in which emitted
light is resonated at a wavelength corresponding to an optical distance
between the reflective electrode and the translucent electrode. A
microcavity relationship is expressed by the following Relational
Expression (1) including phase shifts. Each of a phase shift amount
φ1 on the reflective electrode and a phase shift amount φ2 on the
translucent electrode is normally n. Therefore, when the optical distance
between the reflective electrode and the translucent electrode is set to
an integral multiple of approximately 1/2 of the wavelength, a
microcavity relationship in which light beams reflected between the
reflective electrode and the translucent electrode enhance one another is
obtained. Thus, light extraction efficiency is improved.

[0016] In a case of an actual organic EL device, a viewing angle property
which is in a trade-off relationship with front extraction efficiency is
taken into account, and hence it is not necessary to set exactly the same
thickness as described above.

[0017] Respective portions of the organic EL device according to the
present invention are described in detail. The organic EL device
fundamentally includes an emission layer between a pair of electrodes. In
order to efficiently recombine holes and electrons in the emission layer,
the hole transport layer 3, the electron transport layer 5, the electron
injection layer 6, and a hole injection layer (not shown) are desirably
provided. If provided, the hole injection layer is provided between the
anode and the hole transport layer or the emission layer. The thicknesses
of the layers may be set to form the microcavity as described above or to
reduce power consumption.

[0018] The substrate 1 to be used is normally a glass substrate. The
reflective electrode 2 is desirably made of aluminum, silver, or an alloy
thereof. A thickness of the reflective electrode is desirably in a range
of 50 nm to 300 nm.

[0019] The hole transport layer 3 serves to perform hole injection and
hole transport from the anode (reflective electrode 2 in this
embodiment). If necessary, a hole injection layer (not shown) made of
copper phthalocyanine or vanadium oxide may be formed between the anode
and the hole transport layer 3. Each of the hole transport layer 3 and
the hole injection layer is made of low-molecular and high-molecular
materials having hole injection/transport performance. Examples of such
material include triphenyldiamine derivatives, oxadiazole derivatives,
polyphilyl derivatives, stilbene derivatives, poly(vinylcarbazole),
poly(thiophene), and other conductive polymers, but are not limited
thereto.

[0020] Any known light emitting material may be suitably used for the
emission layer 4. The light emitting material may be a single material
serving as the emission layer 4 or may be a material to be used as a
mixed layer containing a host material and a light emitting dopant or a
charge transport dopant.

[0021] A known material, for example, an aluminum-quinolinol complex or a
phenanthroline compound may be used for the electron transport layer 5.
If necessary, a hole blocking layer of which an absolute value of the
highest occupied molecular orbit (HOMO) energy is large may be formed
between the emission layer 4 and the electron transport layer 5.

[0022] For the electron injection layer 6, there can be used a thin film
(having a thickness of 5 to 10 Å) formed of an alkali (alkaline
earth) metal or an alkali (alkaline earth) metal compound. For example,
lithium fluoride (LiF), potassium fluoride (KF), or magnesium oxide (MgO)
is preferred.

[0023] For the semi-transparent electrode 7, there can be used gold,
platinum, silver, aluminum, chromium, magnesium, or an alloy thereof in a
form of thin film. In particular, a silver thin film or a silver alloy
thin film, which is high in conductivity and reflectance, is desirably
used. A thickness of the translucent electrode 7 is desirably 5 nm or
more to 20 nm or less.

[0024] In the present invention, the inorganic protective layer 8 is
provided in contact with the translucent electrode 7 which is an upper
electrode, and an optical thickness of the inorganic protective layer 8
is defined. The coverage layer 9 and/or the second inorganic protective
layer 10 are/is desirably provided on the inorganic protective layer 8 as
the first inorganic protective layer. Each of the first inorganic
protective layer 8 and the second inorganic protective layer 10 is an
inorganic film made of, for example, silicon nitride (SiN), silicon oxide
(SiO2), indium tin oxide (ITO), or indium zinc oxide
(In2O3--ZnO). When a sputtering method or a CVD method is used,
a dense film which has high moisture resistance may be formed as the
inorganic film. In contrast to this, the coverage layer 9 is made of a
heat or light curable resin, for example, epoxy resin.

[0025] In the present invention, an optical thickness ((refractive
index)x(thickness)) of the first inorganic protective layer 8 is
approximately (2 m+1)/4 (m is natural number) times a maximum peak
wavelength (hereinafter referred to as light emitting wavelength) of a
spectrum of light emitted from the organic electroluminescence device. To
be more specific, assume that "n" indicates the refractive index of the
inorganic protective layer, λ indicates the maximum peak wavelength
of the spectrum of light emitted from the organic electroluminescence
device, and "m" indicates the natural number. In this case, a thickness
"d" of the inorganic protective layer satisfies the following Relational
Expression (2).

[{(2m+1)/4}-(1/8)]λ<nd<[{(2m+1)/4}+(1/8)]λ (2)

The thickness "d" of the inorganic protective layer more desirably
satisfies the following Relational Expression (2a).

[{(2m+1)/4}-( 1/16)]λ<nd<[{(2m+1)/4}+( 1/16)]λ (2a)

The thickness "d" of the inorganic protective layer is optimally
(2m+1)λ/4.

[0026] When the thickness as described above is set, the following
relationship is obtained. That is, light which is reflected at an
interface between the first inorganic protective layer 8 and the coverage
layer 9 and returns to the organic compound layer 11 side is in phase
with light which is reflected on the translucent electrode 7 and returns
to the organic compound layer 11 side. Therefore, the microcavity in the
present invention may be further improved. In another method of improving
the microcavity, the optical thickness nd of the first inorganic
protective layer 8 may be set to approximately 1/4λ. However, in
this case, the first inorganic protective layer 8 becomes thinner and
thus loses a function as the inorganic protective layer. For example,
when a SiN layer which is a normal inorganic film is used as the first
inorganic protective layer, a refractive index of the SiN layer is 2.0, a
maximum peak wavelength of a spectrum of light emitted from a blue
organic EL device having the shortest wavelength is 460 nm, and hence the
thickness is reduced to approximately 58 nm when "nd=1/4λ". Such a
thin film has an insufficient function as the first inorganic protective
layer 8 and a blocking layer for intrinsic moisture in the coverage layer
9. In contrast to this, the thickness which is approximately (2 m+1)/4
times the light emitting wavelength is equal to or larger than 174 nm,
and hence the thin film sufficiently serves as the first inorganic
protective layer 8 in the present invention.

[0027] The order of the optical thickness of the first inorganic
protective layer 8 may be increased from approximately 3/4 times (m=1)
the light emitting wavelength to approximately 5/4 times (m=2) and
approximately 7/4 times (m=3) in order. However, a problem on light
absorption and material consumption due to an increase in thickness
occurs. A problem that a formation time lengthens also occurs. Therefore,
the optical thickness of the first inorganic protective layer 8 is
desirably approximately 3/4 times (m=1) the light emitting wavelength,
that is, the natural number "m" is desirably 1.

[0028] In the present invention, the first inorganic protective layer 8 is
provided to obtain a device protection effect. In order to further
improve the device protection effect, the second inorganic protective
layer 10 is desirably provided on the formed coverage layer 9. In the
present invention, the thick resin film is formed as the coverage layer 9
under the second inorganic protective layer 10, and hence the second
inorganic protective layer 10 may be prevented from causing defects by
unevenness of foreign substances. Another method of preventing the
defects caused by the foreign substances is to thicken the second
inorganic protective layer 10. Particles which are normal foreign
substances are several μm in size. Therefore, when a thickness of
several ten μm is set by using a sputtering method or a CVD method to
cover the unevenness, a tact time lengthens to increase a cost. In
contrast to this, the coverage layer 9 is formed as the resin film for
which an application process may be employed, and hence the coverage
layer 9 which is the thick film may be easily formed. The coverage layer
9 formed as the resin film is desirably provided at a thickness to
sufficiently cover particles of several μm, for example, a thickness
of 5 μm or more to 50 μm or less. The second inorganic protective
layer 10 formed as the inorganic film is desirably provided at a
thickness capable of sufficiently preventing moisture from entering, for
example, a thickness of 0.5 μm or more to 3 μm or less.

[0029] In the present invention, the first inorganic protective layer 8 is
formed under the coverage layer 9. Therefore, even when the coverage
layer 9 is the thick film, the film may be prevented from peeling by
stress when the resin is cured. When the first inorganic protective layer
8 is formed as the dense inorganic film, monomers or solvents may be
prevented from entering the organic compound layer in the case where the
coverage layer 9 is formed, and the organic compound layer may be
prevented from being degraded by intrinsic moisture contained in the
coverage layer 9. The first inorganic protective layer 8 in the present
invention also serves to reduce the stress of the resin of the coverage
layer 9 and protect the device during a printing process.

[0030] In the present invention, when the coverage layer 9 and the second
inorganic protective layer 10 are provided, the thickness of the second
inorganic protective layer 10 is desirably set to a thickness determined
in view of optical interference.

[0031]FIG. 2 is a schematic cross sectional view illustrating a
multi-color display apparatus according to the present invention. In FIG.
2, each of regions surrounded by broken lines corresponds to the organic
EL device illustrated in FIG. 1. Partition walls 12 are provided to
separate the organic EL devices from one another.

[0032] In the multi-color (at least two-color) display apparatus in which
the plurality of organic EL devices are arranged, the first inorganic
protective layer 8 may be patterned corresponding to light emission
colors of the organic EL devices. For process simplicity, the first
inorganic protective layer 8 is desirably provided as a common layer at
the same thickness d1. In this case, the thickness d1 desirably
satisfies Relational Expressions (2) or (2a) with respect to the maximum
peak wavelength λ of a spectrum of light emitted from an organic EL
device of which light emitting efficiency and life property are the
poorest. That is, assume that n1 indicates a refractive index of the
inorganic protective layer 8 which is in contact with the cathode,
λ1 indicates the maximum peak wavelength of the spectrum of
light emitted from the organic EL device of which light emitting
efficiency is the lowest (life property is the poorest), and m1
indicates the natural number. In this case, the thickness d1
desirably satisfies the following Expression (3).

[{(2m1+1)/4}-(1/8)]λ1<n1d1<[{(2m1+1-
)/4}+(1/8)]λ1 (3)

The thickness d1 desirably satisfies the following Expression (3a).

[{(2m1+1)/4}-(
1/16)]λ1<n1d1<[{(2m1+1)/4}+(
1/16)]λ1 (3a)

The thickness d1 is optimally (2m1+1)λ1/4.

[0033] In the present invention, in order to enhance light reflected at
the interface between the first inorganic protective layer 8 and the
coverage layer 9, it is effective to maximize a refractive index
difference at the interface between the first inorganic protective layer
8 and the coverage layer 9. A refractive index of a normal resin is
approximately 1.6. In contrast to this, a refractive index of SiN of a
normal inorganic film is approximately 2.0, a refractive index of
SiO2 is approximately 1.5, and a refractive index of TiO2 is
approximately 2.5. Therefore, the first inorganic protective layer 8 is
made of desirably SiN rather than SiO2, more desirably TiO2. In
a structure in which the coverage layer 9 and the second inorganic
protective layer 10 are not provided on the first inorganic protective
layer 8, an interface opposed to the second electrode (translucent
electrode 7) side of the first inorganic protective layer 8 is an
interface between the first inorganic protective layer 8 and air. A
refractive index of air is approximately 1.0, and hence a refractive
index difference between the first inorganic protective layer 8 and air
is larger than the refractive index difference between the first
inorganic protective layer 8 and the coverage layer 9. Thus, in the
structure, a reflectance at the interface opposed to the second electrode
side of the first inorganic protective layer 8 is large, and hence the
light extraction efficiency is improved.

[0034] In this embodiment, the structure in which the reflective electrode
2 located on the substrate 1 is the anode is described. However, the
present invention is not limited to this structure. A structure may be
employed in which the reflective electrode (cathode) 2, the electron
injection layer 6, the emission layer 4, the hole transport layer 3, the
translucent electrode (anode) 7, the first inorganic protective layer 8,
the coverage layer 9, and the second inorganic protective layer 10 may be
provided in this order from the substrate 1 side.

Example 1

[0035] The organic EL device having the structure illustrated in FIG. 1
was manufactured by the following method.

[0036] An aluminum alloy (AlNd) film was formed as the reflective
electrode 2 on the glass substrate 1 serving as a support member at a
thickness of 100 nm by a sputtering method. Then, an ITO film was formed
at a thickness of 70 nm by a sputtering method. Next, a pixel separation
film made of polyimide was formed at a height of 1 μm and at a taper
angle of 40°. The resultant substrate was sequentially subjected
to ultrasonic cleaning in acetone and isopropyl alcohol (IPA). After
that, the substrate was washed in boiling IPA and dried. The surface of
the substrate was further subjected to UV/ozone cleaning.

[0037] A film of
N,N'-diphenyl-N,N'-di(3-methylphenyl)-1,1'-biphenyl-4,4'-diamine (TPD)
was formed on the substrate 1 at a thickness of 50 nm to obtain the hole
transport layer 3. Next, the emission layer 4 having a thickness of 25 nm
was formed by co-evaporation (at weight ratio of 95:5) of
tris(8-quinolinolato)aluminum (Alq3) and
4,4'-bis(2,2-diphenylethen-1-yl)biphenyl (DPVBi). Then, a film of Alq3
was formed on the emission layer 4 at a thickness of 20 nm to obtain the
electron transport layer 5.

[0038] Next, the electron injection layer 6 was formed at a thickness of
15 nm by co-evaporation of bathophenanthroline and cesium carbonate so
that a cesium concentration in the layer was 8.3 weight %. A film of
silver (Ag) was formed on the electron injection layer 6 by a heating
evaporation method to obtain the translucent electrode 7 having a
thickness of 12 nm.

[0039] Next, a sealing structure in which the first inorganic protective
layer 8, the coverage layer 9, and the second inorganic protective layer
10 are laminated was formed on the translucent electrode 7. First, a SiN
film was formed on the translucent electrode 7 by a CVD method to obtain
the first inorganic protective layer 8 having a thickness of 180 nm.
Then, an epoxy resin film was formed on the first inorganic protective
layer 8 by an application process and heat curing to obtain the coverage
layer 9 having a thickness of 30 μm. Finally, a SiN film was formed on
the coverage layer 9 by a CVD method to obtain the second inorganic
protective layer 10 having a thickness of 1 μm.

[0040] The light emitting wavelength λ of the organic EL device is
equal to 460 nm and the refractive index "n" of the first inorganic
protective layer 8 (SiN) is equal to 2.0, and hence Relational Expression
(2) becomes 143.75≦d≦201.25 in a case where m=1. Therefore,
the thickness "d" of the first inorganic protective layer 8 of the
organic EL device according to this example is within the range defined
in the present invention. As a result, the following relationship is
obtained. That is, light which is reflected at the interface between the
first inorganic protective layer and the coverage layer and returns to
the organic compound layer side is in phase with light which is reflected
on the translucent electrode and returns to the organic compound layer
side. Thus, the microcavity is further improved. The first inorganic
protective layer 8, the coverage layer 9, and the second inorganic
protective layer 10 have sufficient thicknesses, and hence degradation
resulting from moisture, for example, dark spot was not observed.

[0041] When the thickness of the first inorganic protective layer 8 of the
organic EL device according to this example is set to (1/8)λ (that
is, approximately 58 nm), the light extraction efficiency is almost not
changed.

[0042] However, when the thickness becomes thinner, there was a case where
the first inorganic protective layer does not sufficiently serve to
reduce stress and thus film peeling occurs. In addition, there was a case
where a thickness sufficient to block intrinsic moisture contained in the
resin of the coverage layer 9 is not ensured and thus dark spots occur
with time.

Example 2

[0043] A TiO2 film having a thickness of 140 nm was formed as the
first inorganic protective layer 8 by a sputtering method. Next, an epoxy
resin film having a thickness of 30 μm was formed as the coverage
layer on the first inorganic protective layer 8 by an application process
and heat curing. Then, a SiN layer having a thickness of 1 μm was
formed as the second inorganic protective layer 10 by a CVD method. The
other processes were performed as in the case of Example 1 to manufacture
the organic EL device.

[0044] The light emitting wavelength λ of the organic EL device is
equal to 460 nm and the refractive index "n" of the first inorganic
protective layer 8 (TiO2) is equal to 2.5, and hence Relational
Expression (2) becomes 115≦d≦161 in a case where m=1.
Therefore, the thickness "d" of the first inorganic protective layer 8 of
the organic EL device according to this example is within the range
defined in the present invention. As a result, the following relationship
is obtained. That is, light which is reflected at the interface between
the first inorganic protective layer and the coverage layer and returns
to the organic compound layer 11 side is in phase with light which is
reflected on the translucent electrode and returns to the organic
compound layer 11 side. Thus, the microcavity is further improved. The
first inorganic protective layer 8, the coverage layer 9, and the second
inorganic protective layer 10 have sufficient thicknesses, and hence
degradation resulting from moisture, for example, dark spot was not
observed.

[0045] In Example 1, the refractive index difference between the first
inorganic protective layer 8 and the coverage layer 9 is 0.4 (because
refractive index of resin is 1.6 and refractive index of SiN is 2.0). In
contrast to this, the refractive index difference in this example is 0.9
(because refractive index of resin is 1.6 and refractive index of
TiO2 is 2.5) and larger than the refractive index difference in
Example 1. Therefore, the microcavity is further improved. Thus, the
efficiency was improved by 1.08 times as compared with Example 1.

[0046] While the present invention has been described with reference to
exemplary embodiments, it is to be understood that the invention is not
limited to the disclosed exemplary embodiments. The scope of the
following claims is to be accorded the broadest interpretation so as to
encompass all such modifications and equivalent structures and functions.

[0047] This application claims the benefit of Japanese Patent Application
No. 2009-261748, filed on Nov. 17, 2009, which is hereby incorporated by
reference herein in its entirety.